Why are the US highway numbers inverted in Oregon?

I just got back from a road trip around Oregon, Idaho, and Wyoming, and I just noticed something that had never registered with me even though I have lived in the area most of my life: the East-West US highway numbers in Oregon are inverted. East - West US highways start with 2 in the North and increase as you go south. Oregon has US 30 at the top (Astoria), 26 below it (Seaside/Cannon Beach), then 20 (Newport). When you get to Boise, they flip the right way around. Why?